Abstract : Preliminary calculations from a three-dimensional clock model are discussed. In particular, a recent conclusion that short-term stability might be improved by varying the microwave power is considered. Results support the general conclusion, but show that the degree of sensitivity is less than predicted by a one-dimensional model. The difference in the results of the two models is a manifestation of th moe accurate treatment of the position-shift effect in the three-dimensional model. This more accurate treatment is highlighted by the three-dimensional model's determination of isoefficiency contours (contours showing spatial regions in the clock cavity that have equal efficiency for producing clock signal), and noting their spatial dependence upon microwave power. Keywords: Atomic clocks, Rubidium frequency standard, Noise processes.